President Barack Obama has called for the unconditional and immediate release of an Israeli soldier who was apparently captured in a clash with Hamas militants.
Mr Obama said it will be difficult to put back together the ceasefire between Hamas and Israel that collapsed. But he said the US will continue working toward a ceasefire. He said Israel committed to the ceasefire, but the ceasefire was violated.
Mr Obama says it is heartbreaking to see what is happening in Gaza and wants to see everything possible done to ensure Palestinian civilians are not killed.
He warned that Israel must be able to defend itself, but that irresponsible actions by Hamas have put civilians in danger.
The truce was brokered by the US and UN but collapsed less than two hours after it began.
Secretary of State John Kerry denounced as "outrageous" the militant attack that killed two Israeli soldiers and led to the alleged abduction.
"If they are serious about trying to resolve this situation, that soldier needs to be unconditionally released as soon as possible," Mr Obama told reporters at the White House. He said putting the ceasefire back together would be difficult without the release.
"It's going to be very hard to put a ceasefire back together again if Israelis and the international community can't feel confident that Hamas can follow through on a ceasefire commitment," he said.
At the same time, he called the situation in Gaza "heartbreaking" and repeated calls for Israel to do more to prevent Palestinian civilian casualties.
"Innocent civilians caught in the crossfire have to weigh on our conscience, and we have to do more," Mr Obama said.